Production of oligosaccharides in transgenic plants

ABSTRACT

The invention relates to a method for producing oligosaccharides, comprising selecting a gene which codes for an enzyme which is capable of converting sucrose into an oligosaccharide; linking the gene to suitable transcription-initiation and transcription-termination signals in order to provide an expression construct; transforming a suitable plant cell with the expression construct; transforming a suitable plant cell with the expression construct; regenerating a transgenic plant from the transformed plant cell; culturing the transgenic plant under conditions enabling the expression and activity of the enzyme; and isolating the oligosaccharides from the transgenic plant. The invention further relates to the product obtained by means of the method and to the use thereof, in addition to transgenic plants and parts thereof which are capable of producing oligosaccharides.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No. 08/479,470, filed Jun. 7, 1999, now abandoned, which application is incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a method for producing oligosaccharides, to the oligosaccharides produced in this manner, to transgenic plants and plant cells capable of producing oligosaccharides and to the applications of the oligosaccharides obtained in this manner.

BACKGROUND OF THE INVENTION

In the food industry a growing trend toward "light" and low-calorie can be observed. The use of too much fat and/or sugar in products is avoided. To nevertheless be able to provide food products with a sweet taste, an increasing number of sugar substitutes are becoming commercially available. Aspartame is a known example thereof. Aspartame, however, has poor organoleptic properties.

Another type of sugar substitute is formed by oligosaccharides. Oligosaccharides are molecules which consist of two or more monosaccharides such as fructose and/or glucose. The monosaccharides in oligosaccharides are linked to each other either by β--(2-1) or by β--(2-6) bonds. The number of monosaccharides in an oligosaccharide is indicated by means of the DP-value ("Degree of Polymerisation"). A DP-value of 3 means that the oligosaccharide is composed of three monosaccharides. Oligofructoses are oligosaccharides consisting entirely of fructose units. When an oligosaccharide also comprises one or more glucose units these will be linked by means of an α--(1-2) bond to a fructose unit. The composition of oligosaccharides is also designated with the formula G_(m) F_(n), wherein G represents glucose and F fructose and wherein m equals 0 or 1 and n is an integer larger than or equal to 0. Particularly suitable oligosaccharides are those wherein m equals 1 and n is 2 to 8, preferably 2 or 3.

Oligosaccharides can hardly be hydrolysed, if at all, in the human stomach and small intestine. It is known of oligofructose that the digestive enzymes of the human have no effect on the β--(2-1) and β--(2-6) bond in the molecule. They therefore pass through the stomach and the small intestine without being degraded and absorbed into the body. The oligosaccharides do not however leave the body but are metabolised by the microbial flora of the large intestine. Released herein in addition to gas are volatile fatty acids which in turn again serve as an energy source for the intestinal flora. This phenomenon explains why oligosaccharides have a lower energy value for humans than free sugars such as glucose, fructose and sucrose, which are absorbed into the body. Oligosaccharides do however have sufficient sweetening power to serve as sugar substitutes.

It is further known that oligosaccharides, particularly oligofructose, have a bifidogenic effect, that is, that they stimulate the growth of Bifidobacteria in the digestive tract. Bifidobacteria protect against the development of pathogenic bacteria and thereby have a positive influence on health. In addition, oligosaccharides are nutritional fibres.

Different oligosaccharides, which are prepared in diverse ways, are already commercially available.

Oligosaccharides can be made by partial enzymatic hydrolysis of longer vegetable inulin chains. A method herefor is described for instance in European patent application 440.074.

Oligosaccharides can likewise be enzymatically synthesized. For the enzymatic production route use is made of enzymes, fructosyltransferases, which convert sucrose to a mixture of oligosaccharides and which are isolated from different micro-organisms (see Japanese Patent 80/40193).

The known production routes have a number of drawbacks however. Firstly, the known production methods are relatively expensive. In addition to the desired oligosaccharides with a chain length of 2 to about 7, in the produced mixture there also occur a comparatively large number of free sugars and oligosaccharides with a higher chain length. The drawback to many free sugars is that they result in an increase in the energy value of the mixture. Free sugars have for instance an energy content of 17 kilojoule per gram, while pure GF₂ and GF₃ have an energy content of 4 to 6 kilojoule per gram. In addition, free sugars cause dental decay (caries).

Conversely, oligosaccharides with too high a chain length have too little sweetening power, which causes the average sweetening power of the mixture to fall.

In contrast to some other sweeteners, such as for example Aspartame, oligosaccharides have good organoleptic properties.

It is the object of the present invention to provide an alternative production route for oligosaccharides with which the above stated drawbacks are avoided.

SUMMARY OF THE INVENTION

To this end the invention provides a method for producing oligosaccharides, comprising the steps of:

a) selecting a gene which codes for an enzyme capable of converting sucrose into an oligosaccharide;

b) linking the gene to suitable transcription-initiation and transcription-termination signals in order to provide an expression construct;

c) transforming a suitable plant cell with the expression construct;

d) regenerating a transgenic plant from the transformed plant cell;

e) culturing the transgenic plant under conditions enabling the expression and activity of the enzyme; and

f) isolating the oligosaccharides from the transgenic plant.

The invention therefore provides a method with which by means of transgenic plants or plant cells an oligosaccharide or a mixture of oligosaccharides can be produced which have more desirable properties compared with the oligosaccharides prepared by known industrial processes.

The particular advantage of the method according to the invention is that the chain length distribution is narrower, whereby no or few free sugars occur in the end product. The consequence hereof is a lower cariogenicity and the desired lower energy value. There also occur fewer oligosaccharides with a chain length of more than 5. The advantage hereof is that the oligosaccharides produced according to the invention on have a higher specific sweetening capacity. It is the case that the sweetening capacity depends on the "average chain length". The higher the average chain length of a mixture, the lower the sweetening capacity. The advantage of a high specific sweetening capacity is that extra sweeteners hardly have to be added in processing of the product.

A similar consideration applies to solubility. It is also the case here that when the average chain length increases the solubility decreases. The mixtures according to the present invention have a higher solubility than mixtures obtained by means of enzymatic synthesis or enzymatic hydrolysis. In addition, production costs are considerably reduced.

There are indications that short chains can be absorbed better in the bacteria body of Bifidus than long ones. The oligosaccharide mixtures produced by means of the method according to the invention will therefore have a higher bifidogenic effect.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph illustrating the oligosaccharide-producing activity of wild type and modified forms of the Streptococcus mutans fructocyltransferase incubated with sucrose and analyzed on TLC.

FIG. 2 is a photograph illustrating the results of TLC-analysis of transgenic tobacco plants which express the fructocyltransferase gene of Streptococcus mutans.

FIG. 3 is a photograph illustrating the SDS-PAGE gel of purified SST from onion seed.

FIG. 4 is a photograph illustrating the results of TLC-analysis of the reaction products of purified SST from onion seed incubated with sucrose.

FIG. 5 is a graph of the protein elution profile (A280) and the fructocyltransferase activity of two isoforms of 6-SFT obtained from barley.

FIGS. 6A through 6D are chromatograms of 2 isoforms of 6-SFT obtained from barley.

FIG. 7 is a graph of the enzymatic activity of a pool of fractions of 6-SFT after isoelectric focusing under non-denaturing conditions.

FIG. 8 is a photograph illustrating the SDS-PAGE gel after two-dimensional analysis of pool II of FIG. 6.

FIG. 9 is a photograph illustrating the two-dimensional gel electrophoreses of the IEF-markers phycocyanin, beta-lactoglobulin, and bovine carbonic anhydrase.

FIG. 10 is a schematic view of the strategy used to obtain the cDNA clone which codes for 6-SFT from barley.

FIG. 11A illustrates the relationship between FIGS. 11B and 11C.

FIGS. 11B and 11C show the amino acid sequence of 6-SFT from barley compared to different invertases, levanases and levansucrases.

FIG. 12 is a dandrogram of 6-SFT from barley with different invertases, levanases and levansucrases, based on derived immunoacid sequences.

FIG. 13 is a graph illustrating the functional expression of barley 6-SFT in Nicotiana plumbaginifolia protoplasts.

FIG. 14 is a graph illustrating the functional expression of barley 6-SFT in Nicotiana plumbaginifolia protoplasts.

FIG. 15 is a photograph illustrating a native IEF-GEL of a purified enzyme extract of FFT from Helianthus tuberosus L.

FIG. 16 is an HPLC-diagram of tryptic digests of the FFT isoform T1.

FIG. 17 is an HPLC-diagram of tryptic digests of the FFT isoform T2.

In order to select a gene which codes for an enzyme capable of converting sucrose into an oligosaccharide it is possible to search in any possible organism which contains fructosyltransferase activity, for instance micro-organisms such as bacteria, or plants. It is known of many micro-organisms that they contain fructosyltransferases which are capable of producing fructans from sucrose. These enzymes transfer fructose units from sucrose to a fructan acceptor molecule. Microbial fructosyltransferases normally produce fructans with a high DP. The use of a number of fructosyltransferases to manufacture transgenic plants for the production of such polysaccharides is already described in the literature. It is thus known that by incorporating the SacB-gene of Bacillus subtilis in plants the fructan pattern of these plants can be modified (see International Application No. WO 89/12386). This still relates however to the production of high-molecular polysaccharides.

Another gene which is known to code for a fructosyltransferase which can convert sucrose into high-molecular fructans is the ftf gene of Streptococcus mutans. According to the present invention it has now been found surprisingly that in addition to high-molecular weight fructans this fructosyltransferase also produces significant quantities of oligosaccharides in the trisaccharide class (1-kestose). Mutants have also been found which only accumulate trisaccharides and not polysaccharides.

Further known are mutants of the SacB gene of Bacillus subtilis which likewise produce mainly trisaccharides.

A large number of other micro-organisms is likewise capable of fructosyltransferase production. These comprise, but are not limited to, endospore-forming rod bacteria and cocci (for example Bacillus), gram-positive cocci (for instance Streptococcus), gram-negative aerobic rod bacteria and cocci (for instant Pseudomonas, Xanthomonas, Azotobacter), gram-negative facultative anaerobic rod bacteria (for instance Erwinia, Zymomonas), actinomycetes (for instance Actinomyces, Rothia) and cyanobacteria (for instance Tolyporthrix tenuis).

The gene which code for these fructosyltransferases can optionally be modified by targeted or random mutagenesis techniques in order to provide enzymes possessing the desired oligosaccharide-synthesizing enzymatic properties.

Bacterial fructosyltransferases have a relatively low KM for sucrose, approximately 20 mM. The sucrose concentrations in most plants is considerably higher and these enzymes will therefore also be active in plants. An important property of bacterial fructosyltransferase is their activity at low temperatures to 0° C. Plants often come into contact with these temperatures but the bacterial enzymes will still be active even under these conditions.

Fructosyltransferases can also be of vegetable origin. In plants the biosynthesis and degradation of fructan only occur in a limited number of species. Examples are the Asteraceae, Liliaceae and Poaceae families. Starting from the known vegetable fructosyltransferases, the genes suitable for the present invention can be isolated or manufactured either by targeted or random mutagenesis or by selection of already naturally occurring mutants.

An example of a very suitable vegetable fructosyltransferase is the sucrose-sucrose-fructosyltransferase (SST) which occurs in different plant species and which in particular catalyses the synthesis of trisaccharides.

Another example is the sucrose-fructan-6-fructosyltransferase (6-SFT) from barley (Hordeum vulgare L.). According to one embodiment of the invention, transgenic plants are provided which express 6-SFT for the production of oligo-saccharides.

According to another embodiment of the invention, transgenic plants are also provided which contain the fructan-fructan-fructosyltransferase (FFT) of the Jerusalem artichoke (Helianthus tuberosus L.).

For expression in plants of the selected fructosyltransferase gene, transcription-initiation signals such as promotors, enhancers and the like can be added to the gene to obtain the desired expression construct. Such expression promoters can be specific to a special cell type or can be active in a wide diversity of cell types. In addition, the time and site of expression can be determined by use of for instance development-specific promotors. A generally used promotor for gene expression in plants is the 35S Cauliflower Mosaic Virus Promotor (CaMV promotor) which is active in many cell types in the plant depending on the stage of development of the plant. When the fructosyltransferase gene originates from a plant it is also possible to use its own regulatory sequences.

The preferred promotor can be a tissue-specific or constitutive promotor, strong and weak, depending on target plant and purpose. Examples of suitable promotors are the "sink"-specific patatine promotor and the granule-bound starch synthase promotor of the potato, or the sporamine promotor of the sweet potato.

To further increase transcription levels the promotor can be modified and contain an enhancer duplication.

The translation of the mRNAs can be improved by adding a translational enhancer, such as the Alfalfa Mosaic Virus RNA4 translation enhancer signal, which must be present in the transcribed 5' non-translated region.

For correct termination of transcription a terminator sequence can be added to the constructs. An example of such a sequence is the nopaline synthase gene termination sequence.

The choice of expression signals suitable for a specific situation lies of course within the reach of the average skilled person without further inventive work having to be performed for this purpose.

Sucrose, the substrate for the fructosyltransferases, is a carbohydrate present at many different locations. It is synthesized in the cytoplasm and significant quantities can also be found in cytosol, vacuole and the extracellular space (the apoplast) or other possible locations.

Since biochemical processes in plant cells are likewise often limited to a single or a number of cellular compartments, it is desirable to cause the accumulation of the products of the newly introduced genes to take place in a specific compartment. For this purpose targeting sequences which are specific to cellular compartments can be present in the expression construct close to the coding part of the fructosyltransferase genes which are expressed in the transgenic plants. Specific amino acid regions for the targeting to the different cellular locations have already been identified and analysed. These DNA-sequences can be linked to the fructosyltransferase genes such that the enzymatic activity is directed to a desired compartment of the cell or the plant.

In a preferred embodiment of the invention the expression construct therefore also comprises a targeting sequence for directing the fructosyltransferase activity to one or more specific plant cell compartments. Examples of targeting sequences are the signal sequence and vacuolar targeting sequence of the carboxypeptidase Y (cpy) gene, that of patatine from the potato, that of sporamine from the sweet potato, or the signal sequence and apoplastic targeting sequence of the pathogenesis-related protein S-gene (pr-s). These are examples, and the skilled person will be capable of selecting other targeting sequences.

The expression construct can in principe be modified such that targeting takes place to any random cell compartment, such as the vacuole, plastides, cell wall, cytoplasm etc.

It is often advantageous for the plant to control not only the location but also the time of expression of the introduced genes. It is for instance normally desired to limit the expression of the newly introduced enzymatic activities to specific parts of the plant, for instance harvestable organs such as tubers, fruits or seeds. It is moreover often desired to initiate expression in these organs at a particular stage of development. This is certainly the case when the expression of the introduced genes interferes with normal development of such organs.

The oligosaccharides according to the invention can be used as substitutes for sugar, glucose syrup and isoglucose in "light" versions of different food products. Examples of such food products are confectionery, biscuits, cakes, dairy products, baby food, ice cream and other desserts, chocolate and the like. The stimulation of Bifidobacteria is also important for the health of animals. The oligosaccharides according to the invention can therefore also be applied in for instance animal feed.

The present invention will be further elucidated on the basis of the examples hereinbelow, which are only given by way of illustration of the invention and are not intended to limit the invention in any way. Reference is made in the examples to the annexed figures which show the following:

FIG. 1 shows the oligosaccharide-producing activity of wildtype and modified forms of the Streptococcus mutans fructosyltransferase (ftf) which is incubated with sucrose and analysed on TLC as described by Cairns, A. J. and Pollock, C. J., New Phytol. 109, 399-405 (1988). Samples of cultures which were derived from colonies and purified proteins were incubated overnight with 200 mM sucrose in 50 mM sodium phosphate buffer with 1% Triton X-100 at 37° C. Lane 1 shows the reaction products of an S. mutans culture; lane 2 shows the activity of the purified enzyme from S. mutans; lane 3 shows the activity of an E. Coli strain harbouring the plasmid pT102; lane 4 shows the activity of an E. coli strain harbouring plasmid pT2; and lane 5 shows the activity of an E. coli cell which is transformed with the mature S. mutans fructosyltransferase gene under the regulation of an E. coli promotor. The oligosaccharide standards used are in lane A, an extract of an Allium cepa bulb, and in lane H, an extract of a Helianthus tuberosus tuber. In the figure, F represent fructose, G represents glucose, S represents sucrose (disaccharide), N represents neokestose (F2-6G1-2F, trisaccharide), I represents 1-kestose (G1-2F1-2F, trisaccharide), and K represents kestose (G1-2F6-2F, trisaccharide). Higher molecular weight oligosaccharides (DP=4-9) are likewise indicated.

FIG. 2 shows the TLC-analysis of transgenic tobacco plants (KZ) which express the fructosyltransferase gene of S. mutans. Oligosaccharides accumulate in these plants. Lane H shows, as a control, an extract of a Helianthus tuberosus tuber.

FIG. 3 shows the SDS-PAGE gel of purified SST from onion seed. A single band was visible in the SST sample on this gel, stained by means of silver-staining. M represents molecular weight markers wherein their size is indicated in kilodaltons (kD).

FIG. 4 shows the reaction products of purified SST from onion seed which is incubated with sucrose (lanes 4 and 5: O-in vitro). Only trisaccharides are formed. Lane 1 shows the extract of tulip stalks (T), lane 2 shows the extract of Helianthus tuberosus tubers (H), and lane 3 shows the extract of an Allium cepa bulb (O). M represents monosaccharide, S represents sucrose (disaccharide), N represents neokestose (F2-6G1-2F, trisaccharide), and I represents 1-kestose (G1-2F1-2F, trisaccharide). Higher oligosaccharides (DP4-5) are likewise indicated. The products were analysed on TLC as described for FIG. 1.

FIGS. 5 and 6A-D show the separation of 2 isoforms of the sucrose-fructan 6-fructosyltransferase (6-SFT) from barley after the second anion exchange chromatography step on a Resource Q column in a purification procedure. FIG. 5 shows the protein elution profile (A280) and the fructosyltransferase activity of the fractions obtained after chromatography after incubation with 0.2 M sucrose in 25 mM methylpiperazine (HCl) buffer (pH 5.75). The chromatograms (FIGS. 6A through 6D) were obtained by pulsed amperometric detection after anion exchange HPLC separation on a CarboPack-PA100 column. The reaction products were obtained after incubation of pool I and pool III with sucrose alone, or sucrose and isokestose. The carbohydrates were identified by their retention times and trehalose was used as an internal standard.

Open circles in FIG. 5 represent fructosyltransferase activity, which is indicated as the sum of formed kestose, bifurcose, isokestine and kestine. In FIGS. 6A through 6D, p corresponds with a non-identified product resulting from isokestose contaminants, and c with a contamination of the isokestose substrate.

FIG. 7 shows a graph of the enzymatic activity of a pool of fractions of 6-SFT (referred to as pool II; see FIGS. 5 and 6A through 6D) after isoelectric focusing under non-denaturing conditions. Closed triangles indicate beta-fructosidase activity measured as released fructose, while open circles indicate the fructosyltransferase activity measured as formed kestose. FIG. 8 is an SDS-PAGE gel after two-dimensional analysis of pool II after the second anion exchange chromatography. The two 6-SFT isoforms are shown herein. Both isoforms are found to consist of two subunits of respectively 23 kDa and 49 kDa. FIG. 9 is the two-dimensional gel electrophoresis of the IEF-markers phycocyanin (pI 4.6), beta-lactoglobulin (pI 5.1) and bovine carbonic anhydrase (pI 6.0).

FIG. 10 is a schematic view of the strategy used to obtain the cDNA clone which codes for 6-SFT from barley.

FIG. 11 is an overview of the derived amino acid sequence of 6-SFT from barley, different invertases (betafructosidases), levanases and levansucrases. The overview was produced with the Pileup program of the GCG sequences analysis software package. The following abbreviations were used:

    ______________________________________                                         H.v. 6-SFT =                                                                              sucrose-fructan 6-fructosyltransferase from                            barley;                                                                       V.r. Inv = soluble acid invertase from green soya bean                          (mungbean; Arai et al., Plant Cell Physiol.                                    33, 245-252 (1992));                                                          D.c. Inv = soluble acid invertase of carrot (Unger et                           al., Plant Physiol. 104, 1351-1357 (1994));                                   L.e. Inv = soluble acid invertase of tomato (Elliott et                         al., Plant Mol. Biol. 21, 515-524 (1993));                                    D.c. cw Inv = cell wall invertase of carrot (Sturm and Cri-                     speels, Plant Cell 2, 1107-1119 (1990));                                      A.s. Inv = partial invertase sequence of oats (Wu et                            al., J. Plant Physiol. 142, 179-183 (1993));                                  E.c. Inv = invertase (rafD) of Escherichia coli (Aslan-                         dis et al., J. Bacteriol. 171, 6753-6763                                       (1989));                                                                      S.m. Scrb = invertase of Streptococcus mutans (Sato and                         Kuramitsu, Infect. Immun. 56, 1956-1960                                        (1989));                                                                      B.p. LelA = levanase from Bacillus polymyxa (Bezzate et                         al., non-published reference EMBO data base);                                 B.s. SacC = levanase of Bacillus subtilis (Martin et al.,                       Mol. Gen. Genet. 208, 177-184 (1987));                                        K.m. Inu = inulinase of Kluiveromyces marxianus (Laloux                         et al., FEBS Lett. 289, 64-68 (1991));                                        S.c Inv1 = invertase 1 of baking yeast (Hohmann and                             Gozalbo, Mol. Gen. Genet. 211, 446-454 (1988));                               S.o. inv = invertase of Schwanniomyces occidentalis                             (Klein et al., Curr. Genet. 16, 145-152                                        (1989));                                                                      A.n. Inv = invertase of Aspergillus niger (Boddy et al.,                        Curr. Genet. 24, 60-66 (1993));                                               B.a. SacB = levansucrase of Bacillus amyloquefaciens                            (Tang et al., Gene 96, 89-93 (1990));                                         B.s. SacB = levansucrase of Bacillus subtilis (Steinmetz                        et al., Mol. Gen. Genet. 200, 220-228                                          (1985));                                                                      S.m. SacB = levansucrase of Streptococcus mutans (Shiroza                       and Kuramitsu, J. Bacteriol. 170, 810-816                                      (1988));                                                                      Z.m. LevU = levansucrase of Zymomonas mobilis (Song et                          al., non-published reference in EMBO data-                                     base).                                                                      ______________________________________                                    

FIG. 12 is a dendrogram of 6-SFT from barley with different invertases (beta-fructosidases), levanases and levansucrases, based on derived amino acid sequences. The dendrogram was generated with the sequences described in FIG. 11 making use of the Pileup program of the GCG sequence analysis software package.

FIGS. 13 and 14 show the functional expression of barley 6-SFT in Nicotiana plumbaginifolia protoplasts. Error bars indicate the average standard deviation. The 6-SFT cDNA was expressed for 27 hours in protoplasts. Samples were taken a number of times and the fructosyltransferase activity was determined in protoplast extracts by incubation with sucrose (FIG. 13) or sucrose and isokestose (FIG. 14). Open circles show the enzyme activity of extracts of protoplasts which were transformed with the 6-SFT gene construct. Open squares show the activity of extracts of protoplasts transformed with the vector without the 6-SFT cDNA.

FIG. 15 is a native IEF-gel of a purified enzyme extract of fructan-fructan fructosyltransferase (FFT) from Helianthus tuberosus L.. After Coomassie Blue staining there can be seen, in addition to the two most important isoforms of the FFT (T1 (pI 4.45), and T2 (pI 4.75)) a band with a pI of approximately 5.5, which probably corresponds with denatured FFT.

FIGS. 16 and 17 are HPLC-diagrams of tryptic digests of the FFT isoforms T1 (FIG. 16) and T2 (FIG. 17).

EXAMPLE 1 Selection of a Gene

1. Naturally occurring genes.

A large number of microbes were screened for their capacity to produce oligosaccharides from sucrose. For this purpose bacteria cultures were grown overnight in a liquid nutrient. The oligosaccharide-producing activity was determined by incubating a sample of the culture with 200 mM sucrose in the presence of 0.1% Triton X-100. The reaction products were separated by means of TLC and made visible using a fructose-specific reagent (Cairns, A. J. and Pollock, C. J., New Phytol. 109, 339-405 (1988)). It was found as a result of this screening that Streptococcus mutane is an effective producer of oligosaccharides (see FIG. 1). The oligosaccharide-producing enzymatic activity was purified from the Streptococcus mutans culture by means of DEAE-ion exchange chromatography and gel permeation chromatography. It was found herefrom that the enzymatic activity was caused by the product of the ftf gene previously described by Shiroza and Kuramitsu, J. Bacteriol, 170, 810-816 (1988).

The fructosyltransferase (ftf) gene from plasmid pTS102 (Shiroza and Kuramitsu, supra) was subsequently cloned as an EcoRV-BglII fragment in the multiple cloning site of pEMBL9 (Dente et al., Nucl. Acids Res. 11, 1645-1655 (1983)) and expressed from the lacZ promotor present in this plasmid. E. coli was then transformed herewith. The bacteria was hereby made capable of producing oligosaccharides.

The production of oligosaccharides was demonstrated by means of the screening method already mentioned above. Non-transformed E. coli does not produce any oligosaccharides from sucrose.

2. Mutated genes.

By means of mutagenesis it is possible to adapt the oligosaccharide-producing activity of the enzyme as required. Mutations in the gene can be brought about for instance in the following manner.

For mutagenesis of the ftf gene of Streptococcus mutans, the plasmid pTS102 was integrated into the genome of Synechococcus sp. PCC 7942 (R2-PIM9) by means of the genomic integration system (Van der Plas et al., Gene 95, 39-48 (1990), which resulted in strain R2-PTS. This cyanobacteria R2-PTS strain expresses the fructosyltransferase gene. The R2-PTS strain is sucrose-sensitive due to polymer accumulation in the periplasm. An R2-PTS culture was mutated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) which induces point mutations (T→C and G→A mutations). Mutants with a changed fructosyltransferase activity were selected. The culture mutated by means of MNNG was plated on sucrose-containing medium and a total of 400 sucrose-resistant colonies were tested for changed fructosyltransferase activity.

Derived from these colonies were R2-PTS cultures which were concentrated by means of centrifugation. The thus obtained pellets were resuspended in 50 mM sodium phosphate buffer with 1% Triton X-100, 200 mM sucrose and incubated overnight at 37° C. The reaction products were analysed by means of TLC-analysis (Cairns and Pollock, supra). The TLC was developed three times in 85:15 acetone:water and subsequently treated with atomized urea as described by Wise et al., Analytical Chemistry 27, 33-36 (1955). This method preferably stains fructose and fructose-containing polymers.

Of the mutants substantially producing trisaccharides, one was chosen for in vitro demonstration of the enzymatic oligosaccharide-forming activity of the mutated ftf gene in the above described manner.

According to the invention, other mutagenesis methods (site-directed or random) and genes which code for fructosyltransferases from other organisms can likewise be used to select a gene for a mutant oligosaccharide-producing protein.

EXAMPLE 2 Expression of the ftf Gene in Plants

A. Construction of 35S-ftf-NOS in a plant transformation vector.

The plasmid pMOG18 which contains a plant-specific 35S promotor with an enhancer duplication and sequences which stimulate the translations of mRNA is described by Symons et al., Bio/Technology 8, 217-221 (1990). It contains the 35S-promotor-uidA-gene-NOS-terminator construct. A pBluescript II SK-plasmid from Strategene (San Diego, Calif., U.S.A.), from which the internal BamHI-site was removed by digestion with BamHI and filling in the sticky ends with Klenow and ligating once again, was used for further cloning. The 35S-uidA-NOS-fragment was obtained by digestion with EcoRI and Hind-III of pMOG18 and in this BamHI-pBluescript was cloned in the corresponding EcoRI/HindIII site, resulting in plasmid pPA2. Plasmid pPA2 was digested with NcoI and BamHI and the vector-containing fragment was isolated.

The fructosyltransferase gene ftf was cloned from the plasmid pTS102 (see above) as an EcoRV/BglII fragment in the multiple cloning site of pEMBL9. The compatible SmaI- and BamHI locations were used for this purpose. This resulted in the plasmid pTA12.

In order to obtain an NcoI location close to the mature processing site of the ftf gene (nucleotide position 783) (J. Bacteriol. 170, 810-816 (1988)), site-directed mutagenesis was performed as described by Kramer et al. (Nucleic Acids Res. 12, 9441-9456 (1984)) with the following oligonucleotide: 5'-GGCTCTCTTCTGTTCCATGGCAGATGAAGC-3' (SEQ ID NO: 3). Resulting therefrom was plasmid pTD2. At amino acid position +1 (nucleotide position 783) relative to the mature processing site a glutamine was hereby changed into a methionine. The NcoI/PstI fragment, in which the sequence coding for the mature fructosyltransferase is present, was used for further cloning. From this plasmid, the ftf gene was isolated as an NcoI/PstI fragment and this fragment was ligated in the pPA2 vector-containing fragment described above. This results in plasmid pTX. pTX contains the 35S-ftf-NOS-fragment in which ftf shows the mature fructosyltransferase gene without its signal sequence region. pTX was digested with XbaI and HindIII, the fragment containing the complete construct (35S-ftf-NOS) was cloned in the XbaI/HindIII restriction site of pMOG23 (Symons et al., supra) a derivative of the binary plant vector pBIN19 (Bevan, Nucl. Acids. Res. 12, 8711-8721). This resulted in plasmid pTZ.

B. Manufacture and analysis of transgenic plants which express the mature ftf gene.

The pTZ-plasmid was conjugated in Agrobacterium tumefaciens LB4404 (Hoekema et al., Nature 303, 179-180 (1983)) in a three-point crossbreeding making use of the helper plasmid pRK2013 (Lam, Plasmid 13, 200-204 (1985)). The construct was introduced into Nicotiana tabacum var. Petit Havanna (SR1) using the leaf disc transformation method (Horsch et al., Science 227, 1229-1232 (1985)). The regenerated plants were called KP-plants and were selected for kanamycin resistance and cultured on MS medium (Murashige and Skoog, Physiol. Plant. 15, 473-497 (1962)). Thereafter, the plants were grown on soil in the greenhouse and analysed.

The leaf material was cut off and ground in an eppendorf tube. After centrifugation (2 minutes at 16,000 rpm), 1 μl supernatant was analysed on TLC as described in Example 1.

Oligosaccharides were never found in wildtype plants or in plants which were transformed with non-related constructs. The screening of the transformants demonstrated oligosaccharide-accumulating plants using this method (see FIG. 2). The expression levels varied between individual plants which were transformed with the same construct. This is a normal phenomenon in transformation experiments in plants. The variation of the expression levels depends substantially on the integration position in the genome (position effect).

EXAMPLE 3

Oligosaccharide-Producing Enzyme (SST) from the Onion.

In addition to the above used fructosyltransferase genes originating from micro-organisms, such enzymes are also produced by plants. In this example, the SST gene from onion seed is used.

The SST protein from onion seed was purified by chromatographic procedures making use of the following protocol: the seed was incubated at 22° C. between moist cloths for 2 to 3 days and homogenised in 50 mM phosphate-citrate buffer with a pH of 5.7. The starch and debris were centrifuged off at about 10,000 g for 10 minutes. Ammonium sulphate was added to the supernatant to 20%, and the precipitate collected by centrifugation. The concentration of ammonium sulphate in the supernatant was increased to 80%, and the precipitate collected and dissolved in 20 mM NaAc pH 4.6. The solution was dialysed overnight with three buffer changes (20 mM NaAc) and the solution was clarified by centrifugation. The supernatant was placed on an FPLC monoS-column and eluated in 20 mM NaAc pH 4.6 with 0-0.5 M NaCl gradient. After dialysis overnight against 10 mM NaAc pH 5.6, the solution was placed onto a raffinose-epoxy sepharose column. (Pharmacia), which was equilibrated in 10 mM NaAc pH 5.6. Elution took place with a linear gradient consisting of 10 mM NaAc pH 5.6 (buffer A) and 10 mM phosphate-citrate buffer, pH 7.0, plus 0.5 M NaCl-buffer (buffer B). The active fractions were dialysed overnight against 20 mM phosphate-citrate buffer, pH 7.0, and placed on a monoQ FPLC-column in 20 mM phosphate-citrate buffer, pH 7.0. The column was eluated with a gradient of 0-0.5 M. NaCl. For a final purification the protein was placed onto a Sepharose 6-column and eluated with 50 mM phosphate buffer, pH 6.5, with 1% Triton X-100. The silver staining of an SDS-PAGE gel of purified SST from onion seed revealed only one band with a molecular weight of approximately 68,000 d (see FIG. 3).

When this purified SST was incubated with sucrose, only 1-ketose was produced. No significant invertase activity was observed (see FIG. 4).

The amino acid sequence of the purified protein was determined on the basis of peptides obtained by gradual breakdown. On the basis of this information PCR-probes were designed with which the gene coding for the SST of onion seed was isolated. In the same manner as described in Examples 1 and 2, it was demonstrated both in vitro and in vivo that the gene codes for an enzyme capable of producing oligosaccharides.

EXAMPLE 4

Applicability with other Plant Species.

In order to illustrate the general applicability of the technology, the ftf construct described in Example 2 was introduced into different crops. The potato was thus transformed according to the method described in Visser, Plant Tissue Culture Manual B5, 1-9, Kluwer Academic Publishers, 1991. The resulting transgenic plants accumulated oligosaccharides in each tested organ. The same construct was also introduced into the beet (Beta vulgaris L.), which was transformed as described by D'Halluin et al., Biotechnology 10, 309-314 (1992). The resulting transgenic beet plants accumulated significant quantities of oligosaccharides in, for instance, their leaves and roots. The same constructs were introduced into Brassica napus L. which was transformed as according to Block et al., Plant Physiol. 91, 694-701 (1989). The resulting transgenic plants accumulated significant levels of oligosaccharides in, for instance, their leaves and storage organs. It is of course not essential that the plants are transformed in the manner indicated. Other methods within the reach of the skilled person can also be used.

Examples of other plant species which can be modified comprise, but are not limited to, maize (Zea mays L.), wheat (Triticum aestiyum L.), barley (Hordeum vulgare L.), rice (Oryza sativa L.), soya bean (Glycin max L.), pea (Pisum sativum L.), bean (Phaseolus vulgaris L.), chicory (Cichorium intybus L.), sugar cane (Saccharum officinarum L.), sweet potato (Dioscorea esculenta L.), cassava (Manihot esculenta L.) and grasses (for instance Lolium spp., Poa spp. and Festuca spp.).

Plants with natural or induced modified carbohydrate separation patterns can be preferred target plants for the introduction of oligosaccharide-synthesizing genes. Such plants comprise, but are not limited to, natural mutants in starch and sucrose metabolism, and plants in which the starch and sucrose metabolism are modified by means of molecular and genetic techniques, as for instance described in Sonnewald and Willmitzer, Plant Physiology 99, 1267-1270, (1992).

EXAMPLE 5

Sucrose-Fructan 6-Fructosyltransferase (6-SFT) from Barley.

1. Introduction.

Sucrose-fructan 6-fructosyltransferase (6-SFT) is a key enzyme for the biosynthesis of branched fructans (also called graminans) which are typical for grasses. The enzyme forms kestose from sucrose, and bifurcose from sucrose and isokestose. In this example the purification of a 6-SFT from barley (Hordeum volgare L.) is described, in addition to the cloning of the full cDNA and confirmation of the functionality.

2. Purification of Sucrose-Fructan 6-Fructosyltransferase.

Primary leaves of eight to ten day-old barley plants (Hordeum vulgare L. cv Express) were cut off and exposed to light continuously for 48 hours to induce the accumulation of fructans and the enzymes of fructan biosynthesis, as described by Simmen et al., Plant Physiol. 101, 459-468 (1993). The leaves were subsequently frozen in liquid nitrogen and stored at -70° C. until they were used.

An enzyme preparation was prepared by grinding induced primary leaves (700 g fresh weight) to a fine powder in liquid nitrogen and subsequently suspending them in extraction buffer (25 mM methylpiperazine, adjusted to pH 5.75 with HCl, with 1 mM DTT, 1 mM benzamidine, 1 mM EDTA, 0.1mM PMSF and 0.5% PVP). 2 ml per g fresh weight hereof was used. After defrosting, the extract was kept at 4° C. and adjusted to pH 4.75 by adding 0.1 M HCl in drops while stirring. Three hours layer the extract was centrifuged for 30 minutes at 17,000 g. The resulting supernatant was dialysed overnight at 4° C. against dialysis buffer (10 mM methylpiperazine (HCl) buffer (pH 5.75), with 1 mM DTT, 1 mM benzamidine, 1 mM EDTA and 0.1 mM PMSF).

The enzyme solution was purified by means of affinity chromatography on Blue Sepharose. For this purpose the enzyme solution was filtered through a 0.45 micrometer Millipore filter and loaded at a flow speed of 2 ml per minute on a column (26×120 mm) of Blue Sepharose-6-fast flow (Pharmacia, Uppsala, Sweden), which had previously been equilibrated with the above described dialysis buffer. In order to remove proteins without affinity for the dye, the column was washed with three bed volumes of the dialysis buffer. Bound proteins were eluated at a flow speed of 3 ml per minute (5 ml fractions), first with 0.2 M NaCl in 10 mM methylpiperazine (HCl) buffer (pH 5.75) for 30 minutes, followed by a linear gradient of 0.2 M to 0.5 M NaCl in the same buffer within 90 minutes.

All fractions which contained 6-SFT activity were pooled, dialysed overnight at 4° C. against dialysis buffer, and then concentrated to one third of the starting volume by covering the dialysis bag with polyethylene glycol 40,000 and incubating it for 4 hours at 4° C.

For a first anion exchange chromatography step the 6-SFT fraction was filtered and loaded at a flow speed of 3 ml per minute on a 6 ml Resource Q column (Pharmacia), which had been equilibrated earlier with dialysis buffer. After the column was washed with 10 mM methylpiperazine (HCl) buffer (pH 5.75), the bound protein was eluted with a linear gradient of 0 to 0.15 M NaCl in the same buffer within 8 minutes at a flow speed of 15 ml per minute. Fractions of 1 ml were collected. The fractions which contained 6-SFT were pooled and supplemented with ammonium sulphate to a final concentration of 2 M.

The 6-SFT pool was subsequently subjected to hydrophobic interaction chromatography. For this purpose the pool was loaded at a flow speed of 0.5 ml per minute onto an alkylsuperose-column HR5/5 (Pharmacia) which had been equilibrated earlier with 50 mM citric acid-Na₂ HPO₄ buffer (pH 5.0) with 2 M ammonium sulphate. The bound proteins were eluated within 60 minutes at a flow speed of 0.5 ml per minute with a linear gradient of 2 to 0 M ammonium sulphate in 50 mM citric acid-Na₂ HPO₄ buffer (pH 5.0). Fractions of 0.5 ml were collected and the fractions which contained 6-SFT activity were pooled.

The pooled fractions were subjected to gel filtration chromatography and prior thereto first concentrated to a total volume of 190 microliters in microconcentrator centrifuge tubes (Centricon-30, Amicon-Grace, Beverly, Conn.). The concentrate was placed on a Superdex 75 HR 10/30 gel filtration column (Pharmacia), which was equilibrated with 100 mM citric acid-Na₂ HPO₄ buffer (pH 5.75) with 0.2 M NaCl, and eluated with the same buffer at a flow speed of 0.4 ml per minute. Fractions of 0.2 ml were collected and the fractions containing 6-SFT activity were pooled and desalted by 5 successive concentrations and dilution steps in Centricon-30 microconcentrator centrifuge tubes with 10 mM methylpiperazine (HCl) buffer (pH 5.75).

For a second anion exchange chromatography step the desalted sample was loaded onto a 6 ml Resource Q column (Pharmacia). The conditions and buffers were the same as for the first anion exchange chromatography step but the fraction size was reduced to 0.5 ml. The fractions which contained 6-SFT activity were combined in pool I, II and III (FIG. 5).

During purification the enzymatic activity of the fractions was determined after the different purifying steps. For this purpose portions of 50-100 μl of the enzyme preparations were desalted by guiding them over Biogel P-10 columns (8×300 mm) by centrifugation at 350 g for 5 minutes (Simmen et al., supra). Desalted enzyme preparations were incubated with 0.2 M sucrose in 50 mM citric acid-Na₂ HPO₄ buffer (pH 5.75) to identify fractions containing 6-SFT activity during the purification. The final enzyme preparations (pool I and III) were incubated with 0.1 M sucrose alone or in combination with 0.1 M isokestose in 25 mM methylpiperazine (HCl) buffer (pH 5.75). Unless otherwise indicated, the enzyme activity assays were performed for three hours at 27° C. The reaction was stopped by heating the samples for 3 minutes at 95° C. The samples were centrifuged for 5 minutes at 13,000 g, supplemented with trehalose (internal standard) to a final concentration of 0.1 μg/μl, and stored at -20° C. until the analysis.

Neutral carbohydrates were analysed by means of anion exchange chromatography on a CarboPac PA-100 column (Dionex, Sunnyvale, Calif. USA) with a Dionex DX-300 gradient chromatography system coupled to pulsed amperometric detection (Simmen et. al., supra). Prior to analysis by means of anion exchange chromatography, enzyme activities freeing glucose from sucrose were detected in the fractions collected during the enzyme purification using the glucose test kit (GOD-Perid method, Boehringer GmbH, Mannheim, Germany) in accordance with the instructions of the manufacturer.

Two 6-SFT isoforms with indistinguishable catalytic properties were isolated by the purification (see Table I). By affinity chromatography on the HighTrap blue column and by hydrophobic interaction chromatography on the alkylsuperose column, the invertase (beta-fructosidase) activity was almost completely separated from the 6-SFT. This means that 6-SFT has no invertase activity. The mol ratio between beta-fructosidase and fructosyltransferase activity fell by a factor of 6 after affinity chromatography, and was then further reduced to a final ratio of approximately three after hydrophobic interaction chromatography (see Table I). The remaining beta-fructosidase activity could not be separated from 6-SFT and therefore appears to be one of its catalytic properties.

As already demonstrated by Simmen et al., supra, its capacity to transfer fructose to either sucrose or to isokestose is a characterizing property of 6-SFT. Both 6-SFT isoforms which were obtained after the second anion exchange column have the same catalystic properties as shown by HPLC-analysis of the products formed after incubation with sucrose alone or with sucrose and isokestose (FIGS. 6Athrough 6D). In the presence of sucrose as the only substrate, mainly kestose is formed but sucrose is likewise hydrolysed to glucose and fructose. After incubation with sucrose and isokestose, mainly bifurcose is formed and much less sucrose is hydrolysed. This indicates that isokestose is the preferred acceptor compared with sucrose and that the beta-fructosidase activity is a component of the 6-SFT.

3. Gel Electrophoresis.

To illustrate the purity of the two 6-SFT isoforms fractions of the Resource Q chromatography lying between the two 6-SFT peaks, and therefore containing both fractions, were pooled (pool II in FIGS. 5 and 6A through 6D) and analysed by non-denaturing IEF gel-analysis combined with either an enzyme activity assay (FIG. 7) or with SDS-PAGE analysis (FIG. 8).

For two-dimensional electrophoresis of 6-SFT pool II was subjected to isoelectric focussing within a pH range of 4-8 under non-denaturing conditions making use of a Mini-Protean II 2D-cell (Biorad) in accordance with the protocol of the manufacturer.

The 1 mm tubular gels were subsequently either cultured for 30 minutes in 5× sample buffer and loaded onto a 7.5-12% SDS polyacrylamide gel for a separation in the second dimension (Laemmli, Nature 227, 680-685 (1970)), or washed three times for ten minutes in 0.5 M citric acid Na₂ HPO₄ buffer (pH 5.75) and cut into pieces of 2.5 mm for an enzyme activity assay. The 2.5 mm gel pieces were incubated in 0.4 M citric acid Na₂ HPO₄ buffer (pH 5.75) with 0.2 M sucrose and 0.02% NaN₃ for 12 hours at 27° C. After centrifugation at 13,000, g for 5 minutes the supernatant was collected, heated to 95° C. for 3 minutes, supplemented with trehalose (internal standard, final concentration 0.1 μg/μl) and stored at -20° C. for further analysis.

Proteins separated on SDS-polyacrylamide gels were made visible by means of a silver staining (Blum, 1987).

The two isoforms were clearly separated and both had fructosyltransferase and likewise a beta-fructosidase activity. Their pI differed only slightly and was close to pH 5.0. After denaturation both 6-SFT isoforms provided on SDS-PAGE two subunits of respectively 49 and 23 kDa. This data and the almost complete identicity of the fragment patterns obtained by tryptic digestion (data not shown) indicate that the two isoforms display many similarities in structure and sequence. The negatively loaded 6-SFT (containing both isoforms) had a molecular weight of approximately 67 kDa as determined by size-exclusion chromatography (data not shown).

4. Determination of the N-Terminal Amino Acid Sequence.

For N-terminal amino acid sequence determination 100 μg protein of 6-SFT pool I and pool III was loaded onto a gradient gel (7.5-12%) and separated by SDS-PAGE (Laemmli, supra). The proteins were transferred to a polyvinylidene difluoride membrane (Immobilon PVDF transfer membrane, Millipore Corp., Bedford, Mass.) making use of the CAPS buffer system (Matsudeira, J. Biol. Chem 262, 10035-10038 (1987)). The protein bands were made visible on the membrane with 0.2% Ponceau S in 1% acetic acid, cut out and digested with trypsin.

Tryptic peptides were separated by reverse phase HPLC and N-terminal sequence determination of tryptic peptides was performed by automated Edman degradation.

The peptide sequence of the N-terminus of the 49 kDa subunit was determined and both, the large and the small, subunits were digested with trypsin in order to obtain internal peptide sequences. For both subunits two amino acid sequences of tryptic peptides were determined and used to design DNA primers (FIG. 10).

5. Design of a Probe.

A 397 bp fragment was generated by reverse transcription polymerase chain reaction (RT-PCR). For this purpose single-stranded cDNA was synthesized by reverse transcription of Poly(A⁺)-RNA making use of a synthetic oligo-d(T) primer (23 mer) and M-MuLV reverse transcriptase. PCR was performed according to the Perkin-Elmer protocol between the two synthetic, degenerated primers:

(i) CGCCTGCAGGTACCACATGTT(C/T)TA(C/T)CA(A/G)TA(C/T)AA(C/T)CC (SEQ ID NO:4)

(ii) CCACGTCTAGAGCTCTC(A/G)TC(A/G)TACCA(A/C/G)GC(C/G)GTCAT (SEQ ID NO:5)

These primers were designed in accordance with two part sequences of peptides obtained after tryptic digestion of 6-SFT. The resulting PCR product was cloned in the pCR-II™ vector (TA-cloning kit, Invitrogen). Labelling of the fragment with α-³² p-dATP was performed with a random primed labelling kit (Boehringer GmbH, Mannheim, Germany) according to the instructions of the manufacturer.

6. Screening of a cDNA Library.

The fragment of 397 bp generated according to the method of paragraph 5 above was used as a probe in an RNA gel blot analysis of primary leaves, in which the accumulation of fructans was induced by continuous exposure to light for different times. There was found to be no hybridisation signal in the case of untreated leaves while a hybridising band of approximately 1800 bp accumulated rapidly in a manner which corresponded with the increase in 6-SFT activity in the leaves (data not shown). This result points to the presence of a messenger RNA of about 1800 bp in length.

The PCR product was also used to screen a cDNA expression library of primary leaves. A search was made here for a cDNA of full length.

To this end a cDNA expression library was first manufactured by extracting total RNA from 8 day-old cut primary leaves in which the synthesis of fructans was induced by continuous exposure to light for 48 hours. The leaves were ground in liquid nitrogen to a fine powder and suspended in RNA extraction buffer (0.1 M Tris (HCl), pH 9, with 10 mM EDTA, 0.1 M NaCl and 25 mM DTT). The still frozen sample was further ground until a cream-like consistence was reached and the sample was then extracted with phenol-chloroform-isoamylalcohol (25:24:1;v:v:v) (Brandt and Ingversen, Carlsberg Res. Commum. 43, 451-469 (1978). The method was modified somewhat by omitting a second homogenisation step and by precipitating the RNA overnight with 2M LiCl at 4° C. after the last chloroform extraction. After a final ethanol precipitation poly (A)⁺ -RNA was isolated by poly(U)-Sepharose chromatography (Brandt and Ingversen, supra) and used for cDNA synthesis (ZAP-cDNA synthesis Kit, Stratagene, LaJolla, Calif., USA).

The cDNA was ligated in a uni-ZAP-XR vector, digested with EcoRI and XhoI and packaged in phase coats (Gigapack III Packaging Kit, Stratagene, LaJolla, Calif., USA) (7.5×10⁷ plaque-forming units per 5 μg poly(A)⁺ -RNA).

The primary library was screened with the α-³² P-labelled 397 bp long fragment of 6-SFT (see above) at 60° C. in accordance with the Stratagene protocol. Positive clones were screened once again and Bluescript phagemides were finally cleaved from the resulting positive phages using this Exassist/SOLR-system (Strategene, La Jolla, Calif., USA). DNA sequencing of both stands was performed by the dideoxynucleotide sequencing method making use of the sequencing PRO kit (Toyobo, Osaka, Japan). Unless indicated otherwise, standard protocols were used (Sambrook et al., Cold Spring Harbor Laboratory Press, Cold Spring Harbor (1989)). Sequence data-analysis was carried out using the GCG sequence analyses software package, version 7.2 (1992).

After the first screening 9 positive clones were isolated. After a further screening 7 clones remained positive. Of these the sequence was partially determined from the 5' terminus and from the internal primers which were designed on the basis of the PCR product. All 7 clones appeared to code for the same protein, and four of them comprised the complete coding sequence. Of one of the possible clones of full length the sequence was wholly determined on both strands and it was found that it coded for a polypeptide which contained the 49 kDa subunit as well as the 23 kDa subunit (FIG. 10).

The complete nucleotide sequence of the fully sequenced cDNA (SEQ. ID NO:1). It comprises the long open reading frame which begins at nucleotide 46 and ends at nucleotide 1923 for two stop codons. The open reading frame codes for a polypeptide chain of 626 amino acids including a leader sequence of 67 residues in length.

The mature 6-STF starts at nucleotide 246 and therefore has at least 559 amino acid residues with a calculated molecular weight of 61.3 kDa and a calculated pI of 5.37. All 5 of the partial amino acid sequences obtained from the purified protein are present in the amino acid sequence (SEQ ID NO:2) derived from the cDNA. The cDNA likewise contains 45 bp of a 5' non-translated and 171 bp of a 3' non-translated sequence with a poly(A) tail. A possible translation initiation signal (ATG) of the 6-SFT cDNA is localized at the nucleotide positions 46 to 48 and a possible polyadenylating sequence is present at the nucleotide positions 1973 to 1979. It has been found that the mature 6-SFT displays alpha-methyl-mannoside-reversible binding on ConA-Sepharose, which indicates that it is a glycoprotein (data not shown). Similarly, the derived amino acid sequence contains 6 possible glycosylating positions (Asn-X-Ser/Thr).

All peptide sequences obtained from the purified protein are situated without any mismatch in the derived amino acid sequence. The two peptide sequences obtained from the 23 kDa subunit of the purified SFT are localized close to the 3'-terminus of the cDNA, while the sequences obtained from the 49 kDa subunit are localized in the vicinity of the 5'-terminus.

In order to study the possible relation of the cDNA to known beta-fructosidases and fructosyltransferases, the derived amino acid sequence was compared with the sequence of different vegetable, fungal and bacterial invertases, and with bacterial levanases and levansucrases (FIG. 15 and FIG. 16). The cDNA described herein was the highest homology with soluble acid invertases of the green soya bean (mungbean) (Arai et al., supra), carrot (Unger et al., supra), and tomato (Elliott et al., supra), and equally clear homologies with invertases, levanases and levansucrases from other kingdoms, that is, with a number of beta-fructosidases. The comparison of the amino acid sequence indicates at least five well conserved domains. Domains I (SEQ ID NO:6) and IV (SEQ ID NO:7) are less conserved between invertases and levansucrases than domains II (SEQ ID NO:8), III (SEQ ID NO:9) and V (SEQ ID NO:10). With these enzymes domain III in particular is very conserved. Surprisingly, the most limited homology is that with bacterial levansucrases, that is, with a class of enzymes which catalyse a similar 6-fructosyl transfer reaction as 6-SFT (see the dendrogram in FIG. 16).

Expression of 6-SFT in Nicotiana plumbaginifolia Protoplasts.

The 6-SFT cDNA clone was sub-cloned in a derivative of the pUC119 plasmid vector (Samsbrook et al., Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory Press, Cold Spring Harbor (1989) under the regulation of the expression signals of the cauliflower mosiac virus 35S transcript (Neuhaus et al., Proc. Natl. Acad. Sci. USA 88, 10362-10366 (1991)).

Protoplasts of Nicotiana plumbaginifolia were isolated and transformed largely as described by Goodall et al. (Meth. Enzymol. 181 148-161 (1990). In summary, 10 μg of the plasmid containing the 6-SFT cDNA was dispersed in a volume of 10 μl TE buffer in sterile 15 ml plastic tubes. Control transformations were carried out with 10 μg of the same plasmid without insert. 1×10⁶ protoplasts were added up to a volume of 0.5 ml and mixed carefully with an equal volume 20% (w/v) polyethylene glycol 6000. After 2-5 minutes 6.5 ml K3 medium was added and the protoplasts incubated for two hours at 27° C. They were thereafter diluted 1:1 with the W5 osmoticum and pelleted for 10 minutes at 1000 g. All protoplasts (except those which were taken as control at t=0 hour) were resuspended in 2 ml K3 medium and incubated at 27° C. After 3, 6, 9, 18 and 27 hours samples were taken for product analysis. The protoplasts were herein pelleted for 10 minutes at 1000 g after addition of 2 ml W5 osmoticum. The protoplast pellet was resuspended in 0.1 M citric acid Na₂ HPO₄ buffer (pH 5.75), transferred to sterile Eppendorf tubes and frozen in liquid nitrogen. After defrosting the samples were vortexed, and cell debris was pelleted at 13.000 g for 3 minutes. The supernatants (50 to 100 μl) were desalted by guiding them over Biogel P-10 columns as described above. Desalted enzyme samples were incubated with 0.1 M sucrose or with 0.1 M sucrose in combination with 0.1 M isokestose in 50 mM citric acid Na₂ HPO₄ buffer (pH 5.75) with 0.02% NaN₃ for 20 hours at 27° C. The product analysis was performed as described in the case of FIGS. 5 and 6A through 6D after stopping of the reaction by heating the samples at 95° C. for 3 minutes.

After an initial lag-phase of about 3 hours extracts of protoplasts formed kestose from sucrose and bifurcose from sucrose and isokestose. This confirms that the cDNA codes for a functional 6-SFT (FIG. 12). Like the purified enzyme, the activity present in the protoplasts catalysed the production of bifurcose from sucrose and isokestose at a speed that was roughly four times higher than the production of isokestose from sucrose. These results confirm that the cDNA codes for a 6-SFT.

EXAMPLE 6

Fructan-Fructan Fructosyltransferase from Jerusalem Artichoke

Another vegetable fructosyltransferase for application in the invention was purified from Jerusalem artichoke (Helianthus tuberosus L.) by means of the Luscher method (Luscher M. et al., New Phytol. 123, 717-724 (1993)) using salt precipitation, lectin-affinity chromatography and ion exchange chromatography.

The purified enzyme was separated on a native IEF-gel and blotted on a PVDF membrane. The membrane was stained by means of a Coomassie Blue staining and the two most important FFT isoforms (respectively T1 and T2) were cut out (see FIG. 15).

Both proteins T1 and T2 were digested with trypsin and the peptides were separated by means of HPLC. The HPLC-diagrams of the digested FFT isoforms exhibit identical patterns (see FIGS. 16 and 17). The amino acid sequence was determined of two of the purified peptides of T2 (fractions 18 and 24). The sequence of the first peptide was:

NH₂ --E--Q--W--E--G--X--F--M--Q--Q--Y--X--X (SEQ ID NO:11)

The other peptide had the following amino acid sequence:

NH₂ --A--V--P--V--X--L--X--X--P--L--(F/L)--I--X--W--V (SEQ ID NO:12).

In the same manner as in Example 5 the cDNA was isolated and the sequence determined. Using a complete cDNA-clone plane cells were transformed to obtain transgenic plants.

EXAMPLE 7

Use of the Oligosaccharides According to the Invention

The oligosaccharides produced using the method according to the invention can be used as sugar substitutes in different products. Three examples hereof are given below.

1. Ice Cream

Ice cream is prepared from the following ingredients:

    ______________________________________                                         635     parts water                                                              90 parts butter fat                                                            100 parts low-fat milk powder                                                  170 parts oligosaccharides according to the invention                          5 parts Cremodan SE30 ™ (Grindsted)                                         0.3 parts Aspartame ™                                                              flavourings as required.                                              ______________________________________                                    

The milk powder is dissolved in the water. The whole is heated to 40-45° C. The remaining dry ingredients are mixed and dissolved in the warm milk. The melted butter is then added. This whole is then pasteurised for 10 minutes at 72° C. The mixture is thereafter homogenised in a two-stage homogenizer at 150/35 bar. The ice mix obtained is cooled rapidly to 5° C. and the whole is subsequently left to mature for a minimum of 4 hours at 5° C. Finally, the ice mix is aerated and frozen to an overrun of 100%.

After hardening at -35° C. and storage at -20° C. an ice cream is obtained which corresponds in terms of taste and texture with ice cream prepared with natural sugars (saccharose, glucose syrup).

2. Muesli Bar

A muesli bar was prepared from the following ingredients:

    ______________________________________                                         28      parts oligosaccharides according to the invention                        68 parts muesli mix                                                            4 parts cacao                                                                ______________________________________                                    

A syrup was produced from the oligosaccharides by heating, which syrup was mixed with the other ingredients. The bars were formed from the thus obtained mixture in a cylindrical press. Due to the omission of natural sugar the bar is much lower-calorie than conventional bars.

3. Soft Drink

A soft drink was prepared from the following ingredients:

    ______________________________________                                         90      parts water or fruit juice                                               8-10 parts oligosaccharides according to the invention                                artificial sweeteners                                                   flavourings and coloring agents                                                nutrient acid                                                                  carbon dioxide                                                               ______________________________________                                    

All ingredients were dissolved in a part of the water. The remaining water was then added as carbon dioxide-containing water. The energy value of the soft drink is much less because no additional natural sugars are added.

                  TABLE I                                                          ______________________________________                                         Purification of 6-SFT                                                                       fructosyl-       Purifi-                                                                              β-fructosidase/-                        Purification transferase Protein cation fructosyltransferase.sup.b           step     nkatal.sup.c                                                                           %      mg                                                     fold                                                                                                 mol ratio                                                ______________________________________                                         Crude extract                                                                           243     100    5000  1     32                                           Acid 159 66 1700 2 29                                                          precipitation                                                                  High-Trap-blue 71.6 29 450 3 5.7                                               First 22.6 9.3 79 6 6.2                                                        Resource Q                                                                     Alcyl Superose 9.32 3.8 56 4 3.2                                               Superdex 75 6.64 2.7 9.5 15 3.4                                                Second 2.99 1.2 0.6 103 2.7                                                    Resource Q                                                                     pool I                                                                         Second 4.33 1.8 1.7 52 3.2                                                     Resource Q                                                                     pool II                                                                      ______________________________________                                          .sup.a measured as kestoseproducing activity                                   .sup.b mol fructose per mol produced fructose                                  .sup.c nkatal = nmol · s.sup.-1                                 

    __________________________________________________________________________     #             SEQUENCE LISTING                                                    - -  - - (1) GENERAL INFORMATION:                                              - -    (iii) NUMBER OF SEQUENCES: 12                                           - -  - - (2) INFORMATION FOR SEQ ID NO:1:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 2094 base - #pairs                                                 (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: cDNA                                               - -    (iii) HYPOTHETICAL: NO                                                  - -     (iv) ANTI-SENSE: NO                                                    - -     (ix) FEATURE:                                                                   (A) NAME/KEY: CDS                                                              (B) LOCATION: 46..1923                                                - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                - - GCTCAGAATC TACCAAACCC TCTCGGAGTT GACGAGCGGC GCCGC ATG - #GGG TCA              54                                                                                          - #                  - #              Met Gly S - #er                          - #                  - #              1                       - - CAC GGC AAG CCA CCG CTA CCG TAC GCC TAC AA - #G CCG CTG CCC TCG GAC           102                                                                        His Gly Lys Pro Pro Leu Pro Tyr Ala Tyr Ly - #s Pro Leu Pro Ser Asp                  5            - #      10            - #      15                           - - GCC GCC GAC GGT AAG CGG ACC GGC TGC ATG AG - #G TGG TCC GCG TGT GCC           150                                                                        Ala Ala Asp Gly Lys Arg Thr Gly Cys Met Ar - #g Trp Ser Ala Cys Ala             20                 - # 25                 - # 30                 - # 35        - - ACC GTG CTG ACG GCC TCG GCC ATG GCG GTG GT - #G GTG GTC GGC GCC ACG           198                                                                        Thr Val Leu Thr Ala Ser Ala Met Ala Val Va - #l Val Val Gly Ala Thr                             40 - #                 45 - #                 50               - - CTC CTG GCG GGA TTG AGG ATG GAG CAG GCC GT - #C GAC GAG GAG GCG GCG           246                                                                        Leu Leu Ala Gly Leu Arg Met Glu Gln Ala Va - #l Asp Glu Glu Ala Ala                         55     - #             60     - #             65                   - - GCG GGC GGG TTC CCG TGG AGC AAC GAG ATG CT - #G CAG TGG CAG CGC AGC           294                                                                        Ala Gly Gly Phe Pro Trp Ser Asn Glu Met Le - #u Gln Trp Gln Arg Ser                     70         - #         75         - #         80                       - - GGT TAC CAT TTC CAG ACG GCC AAG AAC TAC AT - #G AGC GAT CCC AAC GGC           342                                                                        Gly Tyr His Phe Gln Thr Ala Lys Asn Tyr Me - #t Ser Asp Pro Asn Gly                 85             - #     90             - #     95                           - - CTG ATG TAT TAC CGT GGA TGG TAC CAC ATG TT - #C TAC CAG TAC AAC CCG           390                                                                        Leu Met Tyr Tyr Arg Gly Trp Tyr His Met Ph - #e Tyr Gln Tyr Asn Pro            100                 1 - #05                 1 - #10                 1 -       #15                                                                               - - GTG GGC ACC GAC TGG GAC GAC GGC ATG GAG TG - #G GGC CAC GCC GTG         TCC      438                                                                     Val Gly Thr Asp Trp Asp Asp Gly Met Glu Tr - #p Gly His Ala Val Ser                           120  - #               125  - #               130               - - CGG AAC CTT GTC CAA TGG CGC ACC CTC CCT AT - #C GCC ATG GTG GCC GAC           486                                                                        Arg Asn Leu Val Gln Trp Arg Thr Leu Pro Il - #e Ala Met Val Ala Asp                        135      - #           140      - #           145                   - - CAG TGG TAC GAC ATC CTC GGA GTC CTC TCG GG - #C TCC ATG ACG GTG CTA           534                                                                        Gln Trp Tyr Asp Ile Leu Gly Val Leu Ser Gl - #y Ser Met Thr Val Leu                    150          - #       155          - #       160                       - - CCC AAC GGG ACG GTC ATC ATG ATC TAC ACG GG - #C GCC ACC AAC GCC TCC           582                                                                        Pro Asn Gly Thr Val Ile Met Ile Tyr Thr Gl - #y Ala Thr Asn Ala Ser                165              - #   170              - #   175                           - - GCC GTG GAG GTC CAG TGC ATC GCC ACC CCG GC - #C GAC CCC AAC GAC CCC           630                                                                        Ala Val Glu Val Gln Cys Ile Ala Thr Pro Al - #a Asp Pro Asn Asp Pro            180                 1 - #85                 1 - #90                 1 -       #95                                                                               - - CTC CTC CGC CGG TGG ACC AAG CAC CCC GCC AA - #C CCC GTC ATC TGG         TCG      678                                                                     Leu Leu Arg Arg Trp Thr Lys His Pro Ala As - #n Pro Val Ile Trp Ser                           200  - #               205  - #               210               - - CCG CCG GGG GTC GGC ACC AAG GAT TTC CGA GA - #C CCG ATG ACC GCC TGG           726                                                                        Pro Pro Gly Val Gly Thr Lys Asp Phe Arg As - #p Pro Met Thr Ala Trp                        215      - #           220      - #           225                   - - TAC GAC GAG TCC GAC GAG ACA TGG CGC ACC CT - #C CTC GGG TCC AAG GAC           774                                                                        Tyr Asp Glu Ser Asp Glu Thr Trp Arg Thr Le - #u Leu Gly Ser Lys Asp                    230          - #       235          - #       240                       - - GAC CAC GAC GGC CAC CAC GAC GGC ATC GCC AT - #G ATG TAC AAG ACC AAG           822                                                                        Asp His Asp Gly His His Asp Gly Ile Ala Me - #t Met Tyr Lys Thr Lys                245              - #   250              - #   255                           - - GAC TTC CTC AAC TAC GAG CTC ATC CCG GGC AT - #C TTG CAC CGG GTG GTG           870                                                                        Asp Phe Leu Asn Tyr Glu Leu Ile Pro Gly Il - #e Leu His Arg Val Val            260                 2 - #65                 2 - #70                 2 -       #75                                                                               - - CGC ACC GGC GAG TGG GAG TGC ATC GAC TTC TA - #C CCC GTC GGC CGG         AGA      918                                                                     Arg Thr Gly Glu Trp Glu Cys Ile Asp Phe Ty - #r Pro Val Gly Arg Arg                           280  - #               285  - #               290               - - AGC AGC GAC AAC TCG TCG GAG ATG CTG CAC GT - #G TTG AAG GCG AGC ATG           966                                                                        Ser Ser Asp Asn Ser Ser Glu Met Leu His Va - #l Leu Lys Ala Ser Met                        295      - #           300      - #           305                   - - GAC GAC GAA CGG CAC GAC TAC TAC TCG CTG GG - #C ACG TAC GAC TCG GCG          1014                                                                        Asp Asp Glu Arg His Asp Tyr Tyr Ser Leu Gl - #y Thr Tyr Asp Ser Ala                    310          - #       315          - #       320                       - - GCC AAC ACG TGG ACG CCC ATC GAC CCG GAG CT - #C GAC TTG GGG ATC GGG          1062                                                                        Ala Asn Thr Trp Thr Pro Ile Asp Pro Glu Le - #u Asp Leu Gly Ile Gly                325              - #   330              - #   335                           - - CTG AGA TAC GAC TGG GGA AAG TTT TAT GCG TC - #C ACC TCC TTC TAT GAT          1110                                                                        Leu Arg Tyr Asp Trp Gly Lys Phe Tyr Ala Se - #r Thr Ser Phe Tyr Asp            340                 3 - #45                 3 - #50                 3 -       #55                                                                               - - CCG GCC AAG AAC CGG CGC GTG CTC ATG GGG TA - #C GTC GGC GAG GTC         GAC     1158                                                                     Pro Ala Lys Asn Arg Arg Val Leu Met Gly Ty - #r Val Gly Glu Val Asp                           360  - #               365  - #               370               - - TCC AAG CGG GCT GAT GTC GTC AAG GGA TGG GC - #T TCC ATT CAG TCA GTT          1206                                                                        Ser Lys Arg Ala Asp Val Val Lys Gly Trp Al - #a Ser Ile Gln Ser Val                        375      - #           380      - #           385                   - - CCT AGG ACG GTG GCT CTG GAT GAG AAG ACC CG - #G ACG AAC CTC CTG CTC          1254                                                                        Pro Arg Thr Val Ala Leu Asp Glu Lys Thr Ar - #g Thr Asn Leu Leu Leu                    390          - #       395          - #       400                       - - TGG CCC GTT GAG GAG ATC GAG ACC CTC CGC CT - #C AAT GCC ACG GAA CTG          1302                                                                        Trp Pro Val Glu Glu Ile Glu Thr Leu Arg Le - #u Asn Ala Thr Glu Leu                405              - #   410              - #   415                           - - ACC GAC GTT ACC ATT AAC ACT GGC TCC GTC AT - #C CAT ATC CCG CTC CGC          1350                                                                        Thr Asp Val Thr Ile Asn Thr Gly Ser Val Il - #e His Ile Pro Leu Arg            420                 4 - #25                 4 - #30                 4 -       #35                                                                               - - CAA GGC ACT CAC GCT CGA CAT GCG GAG GCC TC - #T TTC CAC CTT GAT         GCT     1398                                                                     Gln Gly Thr His Ala Arg His Ala Glu Ala Se - #r Phe His Leu Asp Ala                           440  - #               445  - #               450               - - TCC GCC GTG GCT GCC CTC AAC GAG GCC GAT GT - #G GGC TAC AAC TGC AGT          1446                                                                        Ser Ala Val Ala Ala Leu Asn Glu Ala Asp Va - #l Gly Tyr Asn Cys Ser                        455      - #           460      - #           465                   - - AGC AGC GGC GGC GCT GTT AAC CGC GGC GCG CT - #A GGC CCC TTC GGC CTC          1494                                                                        Ser Ser Gly Gly Ala Val Asn Arg Gly Ala Le - #u Gly Pro Phe Gly Leu                    470          - #       475          - #       480                       - - CTC GTC CTC GCC GCC GGT GAC CGC CGT GGC GA - #G CAA ACG GCG GTC TAC          1542                                                                        Leu Val Leu Ala Ala Gly Asp Arg Arg Gly Gl - #u Gln Thr Ala Val Tyr                485              - #   490              - #   495                           - - TTC TAC GTG TCT AGG GGC CTT GAC GGA GGC CT - #C CAC ACC AGC TTC TGC          1590                                                                        Phe Tyr Val Ser Arg Gly Leu Asp Gly Gly Le - #u His Thr Ser Phe Cys            500                 5 - #05                 5 - #10                 5 -       #15                                                                               - - CAA GAT GAG CTG AGA TCG TCA CGA GCC AAG GA - #T GTG ACC AAG CGT         GTC     1638                                                                     Gln Asp Glu Leu Arg Ser Ser Arg Ala Lys As - #p Val Thr Lys Arg Val                           520  - #               525  - #               530               - - ATC GGG AGC ACG GTG CCG GTG CTC GAC GGT GA - #G GCT TTG TCA ATG AGG          1686                                                                        Ile Gly Ser Thr Val Pro Val Leu Asp Gly Gl - #u Ala Leu Ser Met Arg                        535      - #           540      - #           545                   - - GTG CTC GTG GAT CAC TCC ATC GTG CAG GGC TT - #C GAC ATG GGC GGG AGG          1734                                                                        Val Leu Val Asp His Ser Ile Val Gln Gly Ph - #e Asp Met Gly Gly Arg                    550          - #       555          - #       560                       - - ACC ACG ATG ACC TCG CGG GTG TAC CCG ATG GA - #G TCG TAT CAG GAG GCA          1782                                                                        Thr Thr Met Thr Ser Arg Val Tyr Pro Met Gl - #u Ser Tyr Gln Glu Ala                565              - #   570              - #   575                           - - AGA GTC TAC TTG TTC AAC AAC GCC ACC GGT GC - #C AGC GTG ACG GCG GAA          1830                                                                        Arg Val Tyr Leu Phe Asn Asn Ala Thr Gly Al - #a Ser Val Thr Ala Glu            580                 5 - #85                 5 - #90                 5 -       #95                                                                               - - AGG CTG GTC GTG CAC GAG ATG GAC TCG GCA CA - #C AAC CAG CTC TCC         AAT     1878                                                                     Arg Leu Val Val His Glu Met Asp Ser Ala Hi - #s Asn Gln Leu Ser Asn                           600  - #               605  - #               610               - - GAG GAC GAT GGC ATG TAT CTT CAT CAA GTT CT - #T GAA TCT CGT CAT              1923                                                                        Glu Asp Asp Gly Met Tyr Leu His Gln Val Le - #u Glu Ser Arg His                            615      - #           620      - #           625                   - - TAATAAGCTA CATTGGATCA AAGAAGATCA CCAGGGAAGG GCAATTCATA CA -              #TAAATCGA   1983                                                                  - - ATCATTCTGC ACAACCTCGC TTGCAGCATG CATTGAAACA TCTGTATTTG GA -             #TCATCTTC   2043                                                                  - - TTCATTTATG TCATAGTGAA CTATATTACT TTGTAAAAAA AAAAAAAAAA A - #                2094                                                                         - -  - - (2) INFORMATION FOR SEQ ID NO:2:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 626 amino - #acids                                                 (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: protein                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                - - Met Gly Ser His Gly Lys Pro Pro Leu Pro Ty - #r Ala Tyr Lys Pro Leu         1               5 - #                 10 - #                 15               - - Pro Ser Asp Ala Ala Asp Gly Lys Arg Thr Gl - #y Cys Met Arg Trp Ser                    20     - #             25     - #             30                   - - Ala Cys Ala Thr Val Leu Thr Ala Ser Ala Me - #t Ala Val Val Val Val                35         - #         40         - #         45                       - - Gly Ala Thr Leu Leu Ala Gly Leu Arg Met Gl - #u Gln Ala Val Asp Glu            50             - #     55             - #     60                           - - Glu Ala Ala Ala Gly Gly Phe Pro Trp Ser As - #n Glu Met Leu Gln Trp        65                 - # 70                 - # 75                 - # 80        - - Gln Arg Ser Gly Tyr His Phe Gln Thr Ala Ly - #s Asn Tyr Met Ser Asp                        85 - #                 90 - #                 95               - - Pro Asn Gly Leu Met Tyr Tyr Arg Gly Trp Ty - #r His Met Phe Tyr Gln                   100      - #           105      - #           110                   - - Tyr Asn Pro Val Gly Thr Asp Trp Asp Asp Gl - #y Met Glu Trp Gly His               115          - #       120          - #       125                       - - Ala Val Ser Arg Asn Leu Val Gln Trp Arg Th - #r Leu Pro Ile Ala Met           130              - #   135              - #   140                           - - Val Ala Asp Gln Trp Tyr Asp Ile Leu Gly Va - #l Leu Ser Gly Ser Met       145                 1 - #50                 1 - #55                 1 -       #60                                                                               - - Thr Val Leu Pro Asn Gly Thr Val Ile Met Il - #e Tyr Thr Gly Ala         Thr                                                                                              165  - #               170  - #               175              - - Asn Ala Ser Ala Val Glu Val Gln Cys Ile Al - #a Thr Pro Ala Asp Pro                   180      - #           185      - #           190                   - - Asn Asp Pro Leu Leu Arg Arg Trp Thr Lys Hi - #s Pro Ala Asn Pro Val               195          - #       200          - #       205                       - - Ile Trp Ser Pro Pro Gly Val Gly Thr Lys As - #p Phe Arg Asp Pro Met           210              - #   215              - #   220                           - - Thr Ala Trp Tyr Asp Glu Ser Asp Glu Thr Tr - #p Arg Thr Leu Leu Gly       225                 2 - #30                 2 - #35                 2 -       #40                                                                               - - Ser Lys Asp Asp His Asp Gly His His Asp Gl - #y Ile Ala Met Met         Tyr                                                                                              245  - #               250  - #               255              - - Lys Thr Lys Asp Phe Leu Asn Tyr Glu Leu Il - #e Pro Gly Ile Leu His                   260      - #           265      - #           270                   - - Arg Val Val Arg Thr Gly Glu Trp Glu Cys Il - #e Asp Phe Tyr Pro Val               275          - #       280          - #       285                       - - Gly Arg Arg Ser Ser Asp Asn Ser Ser Glu Me - #t Leu His Val Leu Lys           290              - #   295              - #   300                           - - Ala Ser Met Asp Asp Glu Arg His Asp Tyr Ty - #r Ser Leu Gly Thr Tyr       305                 3 - #10                 3 - #15                 3 -       #20                                                                               - - Asp Ser Ala Ala Asn Thr Trp Thr Pro Ile As - #p Pro Glu Leu Asp         Leu                                                                                              325  - #               330  - #               335              - - Gly Ile Gly Leu Arg Tyr Asp Trp Gly Lys Ph - #e Tyr Ala Ser Thr Ser                   340      - #           345      - #           350                   - - Phe Tyr Asp Pro Ala Lys Asn Arg Arg Val Le - #u Met Gly Tyr Val Gly               355          - #       360          - #       365                       - - Glu Val Asp Ser Lys Arg Ala Asp Val Val Ly - #s Gly Trp Ala Ser Ile           370              - #   375              - #   380                           - - Gln Ser Val Pro Arg Thr Val Ala Leu Asp Gl - #u Lys Thr Arg Thr Asn       385                 3 - #90                 3 - #95                 4 -       #00                                                                               - - Leu Leu Leu Trp Pro Val Glu Glu Ile Glu Th - #r Leu Arg Leu Asn         Ala                                                                                              405  - #               410  - #               415              - - Thr Glu Leu Thr Asp Val Thr Ile Asn Thr Gl - #y Ser Val Ile His Ile                   420      - #           425      - #           430                   - - Pro Leu Arg Gln Gly Thr His Ala Arg His Al - #a Glu Ala Ser Phe His               435          - #       440          - #       445                       - - Leu Asp Ala Ser Ala Val Ala Ala Leu Asn Gl - #u Ala Asp Val Gly Tyr           450              - #   455              - #   460                           - - Asn Cys Ser Ser Ser Gly Gly Ala Val Asn Ar - #g Gly Ala Leu Gly Pro       465                 4 - #70                 4 - #75                 4 -       #80                                                                               - - Phe Gly Leu Leu Val Leu Ala Ala Gly Asp Ar - #g Arg Gly Glu Gln         Thr                                                                                              485  - #               490  - #               495              - - Ala Val Tyr Phe Tyr Val Ser Arg Gly Leu As - #p Gly Gly Leu His Thr                   500      - #           505      - #           510                   - - Ser Phe Cys Gln Asp Glu Leu Arg Ser Ser Ar - #g Ala Lys Asp Val Thr               515          - #       520          - #       525                       - - Lys Arg Val Ile Gly Ser Thr Val Pro Val Le - #u Asp Gly Glu Ala Leu           530              - #   535              - #   540                           - - Ser Met Arg Val Leu Val Asp His Ser Ile Va - #l Gln Gly Phe Asp Met       545                 5 - #50                 5 - #55                 5 -       #60                                                                               - - Gly Gly Arg Thr Thr Met Thr Ser Arg Val Ty - #r Pro Met Glu Ser         Tyr                                                                                              565  - #               570  - #               575              - - Gln Glu Ala Arg Val Tyr Leu Phe Asn Asn Al - #a Thr Gly Ala Ser Val                   580      - #           585      - #           590                   - - Thr Ala Glu Arg Leu Val Val His Glu Met As - #p Ser Ala His Asn Gln               595          - #       600          - #       605                       - - Leu Ser Asn Glu Asp Asp Gly Met Tyr Leu Hi - #s Gln Val Leu Glu Ser           610              - #   615              - #   620                           - - Arg His                                                                   625                                                                             - -  - - (2) INFORMATION FOR SEQ ID NO:3:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 30 base - #pairs                                                   (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: other nucleic acid                                          (A) DESCRIPTION: /desc - #= "oligonucleotide"                         - -    (iii) HYPOTHETICAL: NO                                                  - -     (iv) ANTI-SENSE: NO                                                    - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                - - GGCTCTCTTC TGTTCCATGG CAGATGAAGC         - #                  - #                30                                                                       - -  - - (2) INFORMATION FOR SEQ ID NO:4:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 36 base - #pairs                                                   (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: other nucleic acid                                          (A) DESCRIPTION: /desc - #= "synthetic PCR primer"                    - -    (iii) HYPOTHETICAL: NO                                                  - -     (iv) ANTI-SENSE: NO                                                    - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                - - CGCCTGCAGG TACCACATGT TYTAYCARTA YAAYCC      - #                  -      #       36                                                                       - -  - - (2) INFORMATION FOR SEQ ID NO:5:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 35 base - #pairs                                                   (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: other nucleic acid                                          (A) DESCRIPTION: /desc - #= "synthetic PCR primer"                    - -    (iii) HYPOTHETICAL: NO                                                  - -     (iv) ANTI-SENSE: NO                                                    - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                - - CCACGTCTAG AGCTCTCRTC RTACCAVGCS GTCAT       - #                  -      #       35                                                                       - -  - - (2) INFORMATION FOR SEQ ID NO:6:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 47 amino - #acids                                                  (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: peptide                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                - - His Phe Gln Thr Ala Lys Asn Tyr Met Ser As - #p Pro Asn Gly Leu Met       1                5  - #                 10 - #                 15               - - Tyr Tyr Arg Gly Trp Tyr His Met Phe Tyr Gl - #n Tyr Asn Pro Val Gly                   20      - #             25     - #             30                   - - Thr Asp Trp Asp Asp Gly Met Glu Trp Gly Hi - #s Ala Val Ser Arg                    35         - #        40          - #        45                        - -  - - (2) INFORMATION FOR SEQ ID NO:7:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 11 amino - #acids                                                  (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: peptide                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                - - Trp Glu Cys Ile Asp Phe Tyr Pro Val Gly Ar - #g                           1                5  - #                 10                                      - -  - - (2) INFORMATION FOR SEQ ID NO:8:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 8 amino - #acids                                                   (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: peptide                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                - - Ser Gly Ser Met Thr Val Leu Pro                                           1                5                                                              - -  - - (2) INFORMATION FOR SEQ ID NO:9:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 10 amino - #acids                                                  (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: peptide                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                - - Phe Arg Asp Pro Met Thr Ala Trp Tyr Asp                                   1                5  - #                 10                                      - -  - - (2) INFORMATION FOR SEQ ID NO:10:                                     - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 11 amino - #acids                                                  (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: peptide                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:                               - - Asp Trp Gly Lys Phe Tyr Ala Ser Thr Ser Ph - #e                           1                5  - #                 10                                      - -  - - (2) INFORMATION FOR SEQ ID NO:11:                                     - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 13 amino - #acids                                                  (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: peptide                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:                               - - Glu Gln Trp Glu Gly Xaa Phe Met Gln Gln Ty - #r Xaa Xaa                   1                5  - #                10                                       - -  - - (2) INFORMATION FOR SEQ ID NO:12:                                     - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 15 amino - #acids                                                  (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: peptide                                            - -     (ix) FEATURE:  Xaa in position 11 i - #s either phenylalanine        or                                                                                         leucine.                                                              - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:                               - - Ala Val Pro Val Xaa Leu Xaa Xaa Pro Leu Xa - #a Ile Xaa Trp Val          1                5  - #                 10 - #                15              __________________________________________________________________________ 

What is claimed is:
 1. A method for producing oligosaccharides having a low degree of polymerization, comprising the steps of:(a) selecting a gene which codes for a fructosyltransferase enzyme which converts sucrose into an oligosaccharide having a low degree of polymerization; (b) linking the gene to suitable transcription-initiation and transcription-termination signals to provide an expression construct; (c) transforming a suitable plant cell with the expression construct; (d) regenerating a transgenic plant from the transformed plant cell; (e) culturing the transgenic plant under conditions enabling the expression and activity of the fructosyltransferase enzyme; and (f) isolating the oligosaccharides from the transgenic plant.
 2. The method of claim 1, wherein the gene which codes for the fructosyltransferase enzyme which converts sucrose into an oligosaccharide having a low degree of polymerization is of microbial origin.
 3. The method of claim 2, wherein the gene which codes for the fructosyltransferase enzyme which converts sucrose into an oligosaccharide having a low degree of polymerization is the ftf gene of Streptococcus mutans.
 4. The method of claim 1, wherein the gene which codes for the fructosyltransferase enzyme which converts sucrose into an oligosaccharide having a low degree of polymerization is of vegetable origin.
 5. The method of claim 4, wherein the gene which codes for the fructosyltransferase enzyme which converts sucrose into an oligosaccharide having a low degree of polymerization is the sucrose-fructan 6-fructosyltransferase gene from Hordeum vulgare L.
 6. The method of claim 1, wherein the expression construct further comprises at least one targeting signal sequence.
 7. The method of claim 1, wherein the expression construct further comprises at least one enhancer.
 8. A DNA-construct for expressing a bacterial fructosyltransferase enzyme which converts sucrose into an oligosaccharide having a low degree of polymerization in a plant or plant cell, comprising a gene which codes for the bacterial fructosyltransferase enzyme, wherein the gene is mutated so that the resultant oligosaccharide has a low degree of polymerization, coupled in reading frame to plant-specific transcription-initiation and termination signals.
 9. A transgenic plant cell, comprising the DNA-construct as claimed in claim
 8. 10. A transgenic plant, originating from a transgenic plant cell as claimed in claim
 9. 11. Transgenic plant tissue originating from a plant wherein the plant is produced by regeneration from a transgenic plant cell as claimed in claim
 9. 12. A method for producing oligosaccharides having a low degree of polymerization, comprising the steps of:(a) selecting a gene which codes for a bacterial fructosyltransferase enzyme which converts sucrose into an oligosaccharide; (b) mutating the gene so that the resultant oligosaccharide has a low degree of polymerization; (c) linking the mutated gene to suitable transcription-initiation and transcription-termination signals in order to provide an expression construct; (d) transforming a suitable plant cell with the expression construct; (e) regenerating a transgenic plant from the transformed plant cell; (f) culturing the transgenic plant under conditions enabling the expression and activity of the bacterial fructosyltransferase enzyme; and (g) isolating the oligosaccharides from the transgenic plant.
 13. The method of claim 12, wherein the gene selected in step (a) is the sacB gene of Bacillus subtilis.
 14. The method of claim 12, wherein the gene selected in step (a) is the ftf gene of Streptococcus mutans.
 15. The method of claim 12, wherein step (b) further comprises the steps of:(a) integrating the gene into the genome of a bacterium capable of expressing the enzyme; (b) inducing mutations into the gene; and (c) selecting the bacterial mutants with enhanced fructosyltransferase enzyme activity.
 16. The method of claim 15, wherein the gene selected in step (a) of claim 12 is the sacB gene of Bacillus subtilis.
 17. The method of claim 15, wherein the gene selected in step (a) of claim 12 is the ftf gene of Streptococcus mutans.
 18. A method for producing a plant having an increased proportion of oligosaccharides having a low degree of polymerization as compared to the plant in its natural state, comprising the steps of:(a) selecting a gene which codes for a fructosyltransferase enzyme which converts sucrose into an oligosaccharide having a low degree of polymerization; (b) linking the gene to suitable transcription-initiation and transcription-termination signals to provide an expression construct; (c) transforming a suitable plant cell with the expression construct; and (d) regenerating a transgenic plant from the transformed plant cell. 